1. Field of the Invention
The present invention relates to a photoelectric conversion device, and more particularly to a photoelectric conversion device fabricated using a large area process, or a photoelectric conversion device which is suitably applied to a one-dimensional or two-dimensional photoelectric conversion system involving direct reading, such as a facsimile, a digital copy, or an X-ray image pick-up device.
2. Related Background Art
Conventionally, reading systems for the facsimile, digital copy, or X-ray image pick-up device have been known which employ a reduction optical system and a CCD type sensor. By the development of photoelectric conversion semiconductor materials represented by amorphous silicon hydride (hereinafter abbreviated as a-Si), they have been utilized as a so-called contact-type sensor which is formed with a photoelectric conversion unit and a signal processing unit on a large area substrate, and has an optical system for reading at direct magnification from an information source. In particular, a-Si can be used not only as photoelectric conversion material, but also as a field effect transistor (hereinafter denoted as "TFT"), and thus has the advantage of being able to form a photoelectric conversion semiconductor layer and the TFT as a switching element simultaneously.
FIGS. 1A to 1C are views for illustrating photosensors, FIG. 1A and FIG. 1B are typical cross-sectional views for illustrating the layer configuration of two kinds of photosensors, and FIG. 1C illustrates a typical drive method which is common to them. FIG. 1A and FIG. 1B are photosensors of different photo-diode types, referred to as a PIN-type and a Schottky type, respectively. In FIGS. 1A and 1B, 1 is an insulating substrate, 2 is a lower electrode layer, 3 is a p-type semiconductor layer (hereinafter denoted as a p-layer), 4 is an intrinsic semiconductor layer (hereinafter denoted as an i-layer), 5 is an n-type semiconductor layer (hereinafter denoted as an n-layer), and 6 is a transparent electrode layer. The Schottky type of FIG. 1B has a Schottky barrier layer formed to prevent injection of electrons from the lower electrode layer 2 into the i-layer 4 by suitably selecting the material of the lower electrode layer 2. In FIG. 1C, 10 is a symbolized photosensor, 11 is an electric power source, and 12 is a detecting unit such as an ammeter. The direction as indicated by C in the photosensor 10 is toward the side of transparent electrode layer 6, and the direction as indicated by A is toward the side of lower electrode layer 2, wherein the power source 11 is set such that a positive voltage with respect to the A side is applied to the C side.
Herein, the operation will be briefly described. As shown in FIGS. 1A and 1B, if a light is incident from the direction as indicated by the arrow upon the i-layer 4, the light is absorbed therein to produce electrons and holes. Because an electric field is applied to the i-layer 4 by the power source 11, electrons pass through the C side, or the n-layer 5, to the transparent electrode layer 5, while holes move to the A side, or the lower electrode layer 2. Hence, a photo-current will flow through the photosensor 10. Also, when there is no incident light, neither electrons nor holes are produced, and because for the holes within the transparent electrode layer 6, the n-layer 5 acts as a hole injection blocking layer, and for electrons within the lower electrode layer 2, the p-layer 3 of the PIN type of FIG. 1A, or a Schottky barrier layer of the Schottky type of FIG. 1B, acts as an electron injection blocking layer, both electrons and holes can not move, so that no current flows. Accordingly, the current changes depending on whether or not there is incident light, and if it is detected by the detecting unit 12, this photosensor is operable.
However, there were some difficulties to produce a photoelectric conversion device having high S/N ratio and lower cost with the photosensor of the above configuration.